The read/write heads of a hard disk are positioned by DC motors, typically a voice coil motor (VCM), that should be precisely driven for correctly sensing the arm or equivalent holder of the transducers to read data stored along a disk track. Voice coil motors are largely used, and for this reason the ensuing description is referred to these motors, but what will be stated holds even for other kinds of DC motors for positioning read/write heads onto a data storage disk.
Commonly, voice coil motors are current controlled by a feedback loop such as that of FIG. 1, often called Current Minor Loop (CML). The attribute “Minor” means that the CML is an inner feedback loop for controlling the current absorbed by the motor. Conventionally, the main or outer feedback loop is the loop for actually controlling the position of the heads over the hard disk.
In the Current Minor Loop, the current absorbed by the motor is commonly monitored by a sensing resistor in series to at least one winding of the motor. The main drawback of this approach is that the value of the sensing resistors should be precisely determined for minimizing errors, and thus they are quite expensive. The voltage drop on the nodes of a sensing resistor should be provided to an integrated control circuit through dedicated pins. Because of the ever increasing integration level of electronic circuits, a larger number of pins implies larger packaging costs. Moreover, forming feedback loops for controlling in a current mode a motor is a non-negligible cost, especially for large scale productions.
To prevent using dedicated pins for a current sensing signal, a feedback loop and a corresponding controller and an open-loop voltage mode control, such as that depicted in FIG. 2, may be used. The external command U is provided to the controller which generates a respective control voltage of the power amplifier. The parameters of the controller are determined as a function of the parameters of the DC motor.
The admittance of the motor varies during the functioning because the motor heats, and thus its resistance R increases with temperature. As a consequence, the filter that generates the control voltage of the power amplifier may be not sufficiently accurate when the motor resistance differs from its nominal value.
Of course, it is possible to modify parameters that define the filter for adjusting them to resistance variations of the motor, but this can be done only after having determined the effective resistance R of the motor. This should be performed without sensing the current circulating in the motor for having the above mentioned advantages of the open-loop voltage mode control.
A problem that often occurs with this kind of driving mode is that the power amplifier may saturate. As long as the power amplifier is in its linear functioning mode, the voltage control mode has almost the same performances of a current control mode. When the power amplifier saturates, the control of the motor in a voltage mode differs from that in a current mode.
FIG. 3 compares the effects on the motor current of a step voltage that saturates the power amplifier controlled in a voltage mode. The current forced through the motor by the power stage controlled in a voltage mode (Zero Pole Cancellation) differs from the current that would be forced by a power amplifier controlled in a current mode because of the saturation of the power amplifier.